As the field continues to move towards reduced intensity conditioning (RIC) to lower toxicity and immune difficiency, a major obstacle toward more successful allogeneic stem/progenitor cell transplantation is overcoming immune mediated resistance to hematopoietic engraftment in the recipient. During the prior grant period we identified a resistance pathway in recipients sensitized to donor antigens mediated by host CD8 T cells independent of perforin, fasl, TNF and other death receptor ligands. Experiments are designed in the present proposal to understand resistance pathways by effector cells derived from naive (TN) memory CD8 T (TM) cells. Identification and monitoring of an immunodominant tetramer+ host population will enable precise kinetic and compartmental analysis of these cells and studies are proposed to understand the antigen presentation pathway and regulation of resistance by CD4+CD25+ host T regulatory cells. Experiments will determine the relative survival, expansion and function of host TN and TM cells with respect to (RIC) and ablative conditioning protocols. Experiments will test the hypothesis that cytotoxicity plays an important role in T cell mediated resistance in unsensitized recipients but is not required for resistance in recipients previously sensitized to donor antigens. To test the hypothesis that T cells can directly eliminate HSC/PC, ex vivo studies will investigate T cells from cytotoxically normal and deficient recipients co-cultured with HSC and MPP enriched, (LSK FLK-2-),and HSC depleted committed progenitor cell enriched (LSK FLK-2+) populations. Experiments will investigate whether stem/PC are eliminated (i.e., via caspase dependent apoptosis) and/or can be suppressed (i.e., non-apoptotically) in their lineage commitment and differentiation to generate hematopoietic colonies. Overall, these studies will challenge the importance of cytotoxicity in resistance occuring in different transplant settings and generate findings which may discover non-lytic immune inhibition can occur against hematopoietic engraftment by allogeneic HSC/PC.